1. Field of the Invention
The present invention relates to a printing system that includes a first printer that forms an image on continuous-form printing paper and a second printer that receives the continuous-form printing paper fed from the first printer and forms an image on the continuous-form printing paper, and to a printing method.
2. Description of the Related Art
Conventionally, web printing systems (printing systems) have been proposed and brought into practical use as printing system that uses two printers to form images on both sides of elongated print paper (web) that has standard-compliant feed holes. An example of such a web printing system is disclosed in Japanese Patent No. 3680989.
FIG. 9 is a schematic perspective view illustrating basic configuration of a conventional web printing system. Referring to FIG. 9, the web printing system includes two printers P1 and P2 that are arranged in series and that form images on a web by electrophotography. The first printer P1 disposed upstream performs printing on a first surface (front side) of a web W. The web W is delivered from the first printer to a turnover device where the web W is turned upside down and then sent into the second printer P2 disposed downstream of the first printer. The second printer P2 performs printing on a second surface (back side) of the web W. The web printing system configured as described above includes a control device 20 that controls operations of the printer P1 and the printer P2.
FIG. 10 is an exemplary schematic diagram illustrating a configuration of the printer P1, P2 illustrated in FIG. 9. In FIG. 10, a printing process section of a web printing unit that employs electrophotographic method includes a photosensitive member 3. The photosensitive member 3 is configured to transfer and form an electrostatic latent image of a print image to the web W while rotating in a direction the web W is conveyed for printing. Various units (not illustrated) including an electrostatic charging unit, an exposure unit, a developing unit, and a cleaning unit, which are used in forming the electrostatic latent image on the surface of the photosensitive member 3, are provided in the periphery of the photosensitive member 3.
In both lateral-side portions of the web W, standard-compliant feed holes (which engage with the conveying unit 1 and 5, and will be described later), for conveying the web W, are provided continually at regular intervals along the web conveying direction. The web conveying unit 1 is disposed upstream of a transfer device 4 in the web conveying direction, and the web conveying unit 5 is disposed downstream of the transfer device 4 in the web conveying direction. The web conveying units 1 and 5 convey the web W through a transfer region while applying a certain tension on the web W. A web-jam detecting sensor 2 is attached to the web conveying unit 1. The web-jam detecting sensor 2 is configured to detect a jam of the web W.
The web conveying units 1 and 5 convey the web W to a thermal fixing device. The thermal fixing device includes a pre-heating plate 6, a heating roller 7, and a pressure roller 8. The pre-heating plate 6 is a heating plate that preheats the web W so that fixation is performed reliably by the heating roller 7 and the pressure roller 8. A puller roller 9 conveys the web W having undergone fixation to discharge the web W to the outside of the printer through a shooter 10. A buffer mechanism 15 is provided to adjust slack in the web W. The buffer mechanism 15 is configured to be pivotable about its pivot center, which is an end of the buffer mechanism 15 on an upstream side relative to the feed direction of the web W.
FIG. 11 is an exemplary schematic diagram illustrating a configuration of a conventional web conveying unit. Referring FIG. 11, the web conveying unit 1 includes tractor pins 11, which are to be inserted into the standard-compliant feed holes H for conveying the web W, tractor doors 12 having a mechanism that freely opens and closes the tractor doors 12, and web-jam detectors 2a and 2b movable to open and close in synchronization with the tractor doors 12. The above described elements are arranged in a pair. The web-jam detecting sensor 2 may be, for instance, a web-jam sensor of a transmission type; more specifically, the web-jam detecting sensor 2 may be a light-sensitive detector that includes a pair of the web-jam detector 2a that emits sensor light 13 and the web-jam detector 2b that receives the sensor light 13.
In FIG. 11, only portions of the tractor doors 12, which are portions near the web-jam detectors 2a and 2b, are illustrated.
FIG. 12 is a schematic cross-sectional view illustrating how the web-jam detectors 2a and 2b operate. When the web W is jammed at the web conveying unit 1 as indicated by a dotted line in FIG. 12 and causes the sensor light 13 to be shielded by the web W, the web-jam detector 2b does not receive the sensor light 13 any more. The web-jam detector 2b detects a jam of the web W in this manner.
The web-jam detectors 2a and 2b are configured to open and close in synchronization with the tractor doors 12. Accordingly, the web-jam detector 2b detects a web jam even when the web-jam detector 2b is prevented from receiving the sensor light 13 because one of the tractor doors 12 is open or because the tractor door 12 is lifted up by a jam of the web W.
In the web printing system configured as described above, the web-jam detecting sensor 2 detects a jam of the web W irrespective of operations of the first printer P1 and the second printer P2.
However, such an unfavorable situation as described below may occur in the conventional technique described above.
For instance, when printing operation pauses due to a trouble or the like or when printing operation is halted by an operator because of other circumstance, the web W is left as being loaded in the first printer P1 and the second printer P2 in a state where no printing is performed. Under such circumstances, the web W pausing in the thermal fixing device of the first printer P1 continues to be heated by the pre-heating plate 6 and the heating roller 7. Thus, as time passes, the moisture in the web W evaporates, and the web W partially and greatly shrinks and deforms due to heat.
When printing is resumed from such a state as described above, the web W that has the portion greatly deformed by heat shrinkage (hereinafter, “thermally shrunk portion”) is conveyed to the turnover device to be turned upside down and then fed into the second printer P2. When the thermally shrunk portion passes through the web conveying unit 1 of the second printer P2, the tractor door 12 may be lifted up due to the shape of the thermally shrunk portion, disadvantageously causing the web-jam detecting sensor 2 to detect a web jam in some cases.